Continuous filaments or threads are conveyed or delivered in widespread manner e.g. during the manufacture of spunbonded material or webs, in which filaments are drawn by means of air streams from a spinneret and directly deposited to form a web. Numerous methods are known for the production of such spunbonded webs, which differ as to whether the filament bundles are spun from circular spinnerets or the thread or yarn sheets are spun from rectangular spinnerets. They also differ with regard to the production of air streams, their guidance, and generally simultaneous use for depositing the filaments on a collecting belt, so-called web formation.
It is known to spin thread or filament groups from longitudinal or rectangular spinnerets and to supply them in full width to the collecting belt. Filaments combined into bundles have the disadvantage compared therewith that they generally have to be spread out again after the pull-off device, usually a circular injector passage, in order to bring about a uniform depositing of the filaments.
In the production of the air streams drawing the filaments, in accordance with the aforementioned injector passages, such passages are also known for thread or filament sheets. In a rectangular passage, which in its upper part has slots on two facing longitudinal sides, air is forced through the slots and in exceptional cases a different gas; and it flows downward through the passage (German patent 19 65 054). As a function of the slot configuration and the pressure conditions, a specific part of the surrounding atmosphere, usually air, is also sucked in at the top into the passage. The filaments also enter the passage at the top, the secondary air inflow towards the passage proving advantageous for threading purposes. These slotted passages suffer from the disadvantage of requiring a high manufacturing precision, because the slot width are generally less than 1 mm. Additionally, the slotted passages undergo modifications over a period of time due to dirtying or damage, e.g. when removing the dirt. As a result of the adhesion of dirt and of notches in the sensitive slots, which are expensive to manufacture, there are speed differences in the air flowing over the passage width. This leads to a lack of uniformity in the conveyed thread or yarn sheet, so that areas of higher speed guide the threads or yarns to an increased extent, because once a thread is introduced there, it cannot pass again into lower speed areas unless by means of an externally forced lateral movement.
Another method consists of producing the flow by suction instead of forcing the flow in (Textilechnik, 23, 1973, pp. 82-87). Suction takes place below a collecting belt. The filaments drop freely by gravity into a hopper above the belt and are drawn there by the suction flow.
Yet another possible method is to completely encapsulate the space between the spinneret and the passage, and to force the air in laterally from an appropriate distance from the passage above the same, thereby avoiding the described disadvantages of the slot (U.S. Pat. No. 4,340,563). The drawing off passage then comprises a narrowed part of the space encapsulated under an overpressure in the form of two closely facing walls, closed by end walls, which can be parallel to one another, can cross-sectionally widen (diffuser), or can narrow and then widen (venturi).
Another possibility for producing and guiding the airflows drawing or pulling the filaments consists of encapsulating the space between the lower edge of the drawing off passage (or the narrowed part of the above-described encapsulated space functioning as such) and the depositing surface from the surrounding atmosphere and performing a suction process below the depositing surface (German patent 34 01 639). This requires sliding or rubbing seals between the passage end and the depositing surface and between the depositing surface and the suction lines below it. The upper part can also be encapsulated, and in a planned manner, contain air for cooling the threads and for supply to the passage. The area between the spinneret and the passage can also remain open to the atmosphere.
The described apparatus and methods for the production of spunbonded webs admittedly do not have the disadvantage of the time-varying slot geometry, but suffer from a limitation towards higher speed levels. High air speeds are required in order to bring about an optimum high molecular orientation of the filaments by deformation below the spinneret up to the solidification thereof. This is particularly the case with polyester threads, whereas in the case of polypropylene air drawing alone does not lead to high strength and limited elongations, as permitted by polycondensates, which are not required in most uses in the hygienic and medical fields. In the case of polypropylene geotextiles, higher strengths than hitherto are desired and all that remains is additional mechanical stretching, where once again air streams can perform the web formation. Higher thread speeds are also desired in connection with increasing the throughput and improving the economics of the method. However, there are limits thereto in web formation. The higher the air speed, the greater the turbulence between the drawing off passage and the collecting belt. In addition, ambient air is sucked in through the free jets below the drawing off passage. There is an increased air exchange prejudicial to the overall method and further turbulence is produced by the mixing action. In addition, the high degree of delay of the flow leaving the passage on the way to the collecting surface resulting from stagnation leads to a high irregular flow level, partly with a back-flow and an increased turbulence of the threads from the depositing surface. Apart from the increased energy costs, turbulence, in particular, leads to non-uniform thread distribution through the formation of bundles and strands in the subsequent web.
The widening of the air passage or duct following its narrowest region, where the highest air speeds prevail, exerting the highest forces on the threads, can bring about a diffuser which also causes serious problems. Such delayed flows, particularly following onto an upstream flow in the narrow part of the passage do not permit a strong delay through the boundary layers formed there without the flow breaking away. Such breaking away or detachment effects produce turbulence and this in turn leads to the agglomeration of the filaments so as to form strands and bundles. In addition, such detachments are generally not continuous flows, which then leads to a non-uniform thread distribution in the web.
It is particularly difficult to achieve a detachment-free flow in a planar diffuser, because these disturbances emanate from the end wall boundary layers and there is frequently a one-sided detachment on the longitudinal faces with an oblique flow profile and therefore a non-uniform distribution of the threads over the passage. In the case of circular diffuser passages the conditions are somewhat better, but here again only a limited widening can be achieved and there is a maximum 7.degree. widening angle.